Dashiel,
AFAIK, those kind of fully customized editors are only available to unmanaged adapters (yes, several of the native adapters shipping with biztalk are still written in unmanaged code, such as the FILE and SMTP adapters). However, for managed adapters you can still tweak quite a bit of what you get with the default property grid based property dialog (including adding a popup WInForms dialog when you edit a specific property which can have anything you want in it).